I can do 12psi at 4K rpm with the twins. My old 62 was 0

 

differences are in guages, and guage quality. not all guages will read exactly the same, even of the same make and model.

everyone has mentioned a factor, all are factors. none will ever add up to be the same.

don't worrry about it.

 

Maybe I need to get my valves adjusted.

I don't think it's the gauge. A quality gauge (especially mechanical) should have accurate readings. It's scary to think that your pyrometer gauge could be off.

Infidel, so you're saying an exhaust restriction like a plugged cat could cause the engine not to boost in neutral?

For all you guys not making boost in neutral, you can still hear the turbo spool up right?

 

mechanical guages even quality guages have a variance.

a pluged CAT is a restriction after the turbo. your drive pressure will be very high, and the turbo will not be driven due to lack of flow. no boost.

boost is a direct function of load on the engine. why are we worring about not making any in neutral anyways?

 

You're right about gauges being off. But 5 psi on a 35 psi boost gauge is almost 15% error. A good gauge would have 1-2% error.

 

granted, yes. but look at it like this. say its a 60 or 80 or 100 psi guage. like some of the big boosters run. and one guy is +2% and the next is -2% then take into the account the differences in trucks and fueling systems add ons. actual physical components. and maybe these people have intake leaks that are small enough to pass a leakdown test, but won't build fast enough on your free wheel spike.

like willy91 for example again. he has my old twins on his truck now and i never made 12 psi in neutral with them. but its a different truck altogether. than mine was.

and bingo dartmouth 12v has it nailed. turbo = thermodynamic device. must have load to add fuel to make more boost. the CAC pipes and engine is the restriction. that dimensionally never changes. must increase load to make boost. whether you make 0 3 6 or 12psi on a no load free rev. it doesn't matter because you're not putting the turbo to work.

I'm thinking that a combination of pinhole leaks, guage variances, truck setups all contribute to some yes some no.

 

A turbo is driven more by RPM than heat if you ask me. I know others will disagree, but You can get your EGT's at 2000 degrees at 1k rpm's and i still bet your turbo wont spool. 600* at 1600rpm will spool it right up though. Its the volume of air coming into the exhaust housing. I know the hotter air is, the more it expands, but the more RPM you are turning, the more air you are flowing through the motor. Gassers can spool big turbos easily because they have the RPM to spin them.

 

A turbo is a thermodynamic device. It is driven by the amount of energy coming out of the exhaust in the form of heat. EGT's are not the same as the thermal energy exiting the engine. At lower rpms, you have less airflow so you would need much hotter egt's to get the same amount of energy. At higher rpms, you are flowing a lot of air out of the exhaust so it doesn't need to be as hot to have the same amount of energy.

If the turbo was based on rpm, then you would be able to get the same amount of boost with no load on the truck as you could with a load. If rpm=boost, everyone would reply that they could hit 20psi of boost during a free rev.